This invention relates generally to radio-frequency (RF) antennas, and, in particular, to convoluted and folded antenna patterns based upon generalized Hausdorff structures.
The design of RF antennas can be exceedingly complex and mathematically and empirically intense due to the wide range of tradeoffs involving frequency response, sensitivity, directionality, polarizations, and so forth. Conventional antennas, such as open loops and parallel element arrays are limited in terms of applicability, such that, quite often, a particular geometry is relegated to a dedicated frequency band or direction.
It has been found that so-called fractal antennas offer certain advantages over conventional designs, including smaller size and desirable performance at multiple frequencies. In addition to greater frequency independence, such antennas afford enhanced radiation, since the often large number of sharp edges, corners, and discontinuities each act as points of electrical propagation or reception.
The term xe2x80x98fractalxe2x80x99 was coined by Benoit Mandelbrot in the mid-70s to describe a certain class of objects characterized in being self-similar and including multiple copies of the same shape but at different sizes or scales. Fractal patterns and multi-fractal patterns have by now been widely studied, and further information on fractal designs may be found in Frontiers in Electromagnetics, IEEE Press Series on Microwave Technology and RF, 2000, incorporated herein by reference.
Fractal antennae were first used to design multi-frequency arrays. The Sierpinski gasket antenna, which resembles a triangle packed with differently sized triangles of the same general orientation, was the first practical antenna to maintain performance at several (5) bands. Other fractal geometries used in antenna design include the Sierpinski carpet, which may be viewed as a square-within-a-square version of the Sierpinski gasket, as well as the snowflake or Koch curve, which has also been used in monopole form.
In designing an antenna based upon a folded or convoluted fractal-type geometry, the resonance frequencies may be a function of multiple parameters, including the shape of the structuring or replicated element, the size of the smallest element, and the number of scaling factors used simultaneously in the pattern. Despite the improved performance of antennas based upon fractal geometries, existing designs exhibit certain disadvantages. In particular, though self-similar, conventional fractals are based upon a heterogeneous reproduction of structuring elements limited to transformations in terms of rotation, translation, and scale, fractal structures utilize a subset of a Hutchinson operator W, wherein a regular shape, such as a triangle or square is iterated such that the same behavior may be obtained, albeit at multiple frequencies. Increased degrees of freedom are required to design structures with appropriate gain, beam patterns, polarization response, and other desirable characteristics.
This invention improves upon the existing art by providing an approach to antenna design that optimizes gain, beam pattern, polarization response, or other qualities, including desirable characteristics which might otherwise be in conflict. Broadly, the invention endeavors to generalize self-replicating antenna patterns through the use of additional transformations and candidate geometric shapes. One improvement over the self-similar fractal structure is the self-affine structure, which, in addition to fractal-type operators permits transformations such as skewing, reflection (i.e., flipping). In the preferred embodiment, however, Hausdorff structures are used, including multiple instances of variable patterns to enhance variability and design freedom.
More particularly, whereas the self-affine structure utilizes a single Hutchinson operator, according to one use of the Hausdorff structure consistent with this invention, different Hutchinson operators are utilized to realize xcex3n-arbitrary different radiation patterns, including patterns optimized for multiple frequencies. Although a more limited Hausdorff structure may be based upon a single type of geometry (i.e., a square, triangle, or arc), the most preferred more generalized approach according to the invention applies a sequence of different Hutchinson operators to different geometric subsets, thereby achieving patterns which are not only arbitrary in terms of wavelength/frequency, but also permit variable radiation patterns and variable polarization other desirable criteria inherit in the approach. In addition to the use of variable scaling, geometric patterns, and the like, multiple structures may be placed within the same spatial footprint to permit reception over more bands.
According to yet a further aspect of the invention, a dynamic reconfigurable antenna array is provided, enabling a single device to be simultaneously tuned to different or multiple frequencies or other response criteria. The antenna array may be made directional in its radiation (or reception) pattern either by changing the configuration of the array, changing the feed points in the array, or electrically steering the pattern using standard beam formatting techniques on multiple taps. Although the resultant geometry is preferably that of a generalized Hausdorff structure, it will be appreciated by those of skill that this aspect of the invention is applicable to any type of antenna geometry, including conventional, fractal, self-affine, and so forth.
In a reconfigurable implementation, once a particular antenna architecture is defined, switches are placed at key points of the structure enabling the pattern to be changed dynamically. Such switching may be carried out in real time in accordance with transmissions/reception characteristics, though a more preferable approach is to change the pattern and verify the simulations in accordance with the switched elements. The switches may be implemented with any appropriate technology, including electrical switches such as MOSFETs, though, in the preferred embodiment, MEMS mechanical switches are used to ensure that the resulting pattern includes continuous metalization for the least amount of leakage and unwanted artifacts.
As an alternative to a fixed pattern with switches used to swap elements or change feed points, a reconfigurable multi-dimensional array may be used having an active area optimized to maximize reception for a desired frequency and/or direction. This aspect of the invention may exploit flat-panel technology, wherein, for example, a transparent conductor array xe2x80x98facexe2x80x99 may be mapped to an addressable interconnect back plane to achieve a desired level of reconfigurability.